FIG. 51 is a simplified cross-sectional view showing a tag 1 according to a conventional technique. FIG. 51 shows the case of wireless communication using an electromagnetic induction system typically used for a 13.56 MHz band. An RFID (radio frequency identification) system is a system used for automatically recognizing a solid matter, and basically is provided with a reader and a transponder. As the transponder of this RFID system, the tag 1 is used. The tag 1 has a coil antenna 2 that is a magnetic field-type antenna detecting lines of magnetic force, and an integrated circuit (IC) 3 that is used for performing wireless communication using the coil antenna 2. In the tag 1, at the time when a request signal from the reader is received, information stored in the IC 3 is sent, that is, the reader is allowed to read information held in the tag 1. For example, the tag 1 is attached to a product, and used for management of products such as prevention of product theft or recognition of inventory status.
When a communication jamming member 4 (a conductive material in this example) is present in the vicinity of the antenna 2, for example, when the tag 1 is attached to a metal product in use, lines of magnetic force of a magnetic field that is formed by electromagnetic wave signals sent and received by the antenna 2 pass through points in the vicinity of the surface of the communication jamming member 4. In this case, an eddy current is formed at the communication jamming member 4, and electromagnetic wave energy is converted into thermal energy and absorbed. When the energy is absorbed in this manner, electromagnetic wave signals are significantly attenuated, which makes it impossible for the tag 1 to perform wireless communication. Furthermore, when the induced eddy current generates a magnetic field (diamagnetic field) in the orientation opposite to the magnetic field for communication of the tag, a phenomenon occurs in which the magnetic field is cancelled. This phenomenon also makes it impossible for the tag 1 to perform wireless communication. Furthermore, due to the influence of the communication jamming member 4, a phenomenon occurs in which the resonance frequency of the antenna 2 is shifted. Accordingly, the tag 1 cannot be used in the vicinity of the communication jamming member 4.
FIG. 52 is a simplified cross-sectional view showing a tag 1A according to another conventional technique. The tag 1A shown in FIG. 52 is similar to the tag 1 in FIG. 51, and thus the corresponding constituent elements are denoted by the same numerals, and only different constituent elements in the configuration will be described. In order to solve the problem of the tag 1 in FIG. 51, the tag 1A in FIG. 52 is configured to include a magnetic wave absorbing plate 7 disposed between the antenna 2 and the member 4 that is a product to which the tag 1A is attached. The magnetic wave absorbing plate 7, which is a sheet having a complex relative magnetic permeability, is made of a highly magnetically permeable material such as sendust, ferrite, or carbonyl iron, that is, a material having a high complex relative magnetic permeability.
The complex relative magnetic permeability has a real number part and an imaginary number part. When the real number part becomes high, the complex relative magnetic permeability becomes high. In other words, a material having a high complex relative magnetic permeability has a high real number part in the complex relative magnetic permeability. In a case where a material having a high real number part in the complex relative magnetic permeability is present in the magnetic field, lines of magnetic force concentratedly pass through the material. In the tag 1A that uses the magnetic field-type antenna 2 detecting lines of magnetic force, leakage of the magnetic field to the communication jamming member 4 is prevented by arranging the magnetic wave absorbing plate 7. Thus, even in the vicinity of the communication jamming member 4, the tag 1A can perform wireless communication while suppressing attenuation of magnetic field energy. This sort of tag 1A has been disclosed in, for example, Japanese Unexamined Patent Publication JP-A 2000-114132.
In another conventional technique, a sheet member is attached via an adhesive or the like to a non-contact wireless data carrier that is disposed near a wall face made of a metal or the like and that can send and receive predetermined radio waves, and thus this sheet member absorbs radio waves oriented toward the wall face or radio waves reflected by the wall face, thereby making it possible to send and receive data in the entire space in a radio wave area effective for the operation of the non-contact wireless data carrier. This example is for the RFID system in wireless communication using a radio wave method in a 2.4 GHz band. Furthermore, the non-contact wireless data carrier, a spacer that has a predetermined thickness and that does not absorb radio waves, and a radio wave reflecting member are attached to each other via an adhesive or the like, and the thickness of the spacer 8 is set so that the position of the non-contact wireless data carrier does not match a position away from the radio wave reflecting member by λ/4 (λ denotes the wavelength) or a position away from that position by nλ/2 (the symbol n denotes a natural number), thereby making it possible to send and receive data in the entire space in a radio wave area effective for the operation of the non-contact wireless data carrier. A data carrier system using the non-contact wireless data carrier has been disclosed, for example, in Japanese Unexamined Patent Publication JP-A 2002-230507.
A communication jamming member in the invention refers to a member that may deteriorate communication properties of an antenna when the communication jamming member is present in the vicinity of the antenna, compared with the case of a free space. The communication jamming member corresponds to, for example, conductive materials such as metals, dielectric materials such as glass, paper, and a liquid, and magnetic materials having magnetic properties. In a case where a conductive material is present in the vicinity of an antenna element, the input impedance of the antenna element is significantly lowered, and thus wireless communication becomes difficult. Moreover, a dielectric material such as cardboard, a resin, glass, or a liquid jams wireless communication because the dielectric constant of the dielectric material lowers the resonance frequency of the antenna. Furthermore, a magnetic material also jams wireless communication because the magnetic permeability of the magnetic material lowers the resonance frequency of the antenna.
In a case where the magnetic field-type antenna 2 such as a coil antenna is used as in the tag 1A shown in FIG. 52, leakage of a magnetic field is prevented, and thus wireless communication can be performed in the vicinity of the communication jamming member 4. However, this configuration has the problem that a sufficient communication distance cannot be typically secured with a magnetic field-type antenna. Furthermore, it is considered that this sort of configuration for preventing leakage of a magnetic field is not effective for a case in which an electric field-type antenna detecting lines of electric force is used, and the application thereof has not been investigated.
In JP-A 2002-230507, the radio wave reflecting member is overlaid via a sheet member or a spacer on the non-contact wireless data carrier, and thus the position of the data carrier is set so as not to match a position away from the radio wave reflecting member by λ/4 or a position away from that position by nλ/2 (n is a natural number). JP-A 2002-230507 describes that a point where data cannot be sent or received due to mutual cancellation of incident waves and reflected waves appears in each point away from the reflecting face by λ/4 and point away from that position by λ/2. However, as shown in FIG. 12 by the present inventors, the phase of radio waves is shifted by 180° when the radio waves are reflected by the radio wave reflecting face, and thus the position away from the radio wave reflecting face by λ/4 has the largest electric field intensity due to interference. At the same time, the magnetic field intensity at this position becomes zero. That is to say, although data cannot be received by a magnetic field-type antenna, data can be received optimally by a commonly used electric field-type antenna. Thus, in a case where this position is not included, there is the problem that a sufficient communication distance cannot be secured in the vicinity of the communication jamming member.
The problem in the shift of the resonance frequency is that since the shift varies depending on a material (material quality) that is present in the vicinity, the shift amount is not constant, and thus a measure for improving communication (modifying resonance frequency) is individually required.